1. Field of the Invention
This invention relates to a speed variator for controlling speed in an AC or universal motor supplied from an AC supply source to regulate the motor speed at a predetermined speed.
2. Description of the Prior Art
More especially, such a speed variator comprises a controllable switching member having a control electrode, and a circuit for regulating the motor speed at a predetermined speed. The regulation circuit includes an adjustable resistor and a capacitor, both connected in series across two terminals of the switching member and having a common terminal connected to the control electrode. Generally, the switching element is a triac, which is equivalent to two antiparallel-connected thyristors, the two triac terminals constituting speed variator terminals.
A main disadvantage of such a speed control circuit is that the motor speed decreases when load torque on the motor increases. This decrease in the speed thus disturbs the use of the constant speed motor preadjusted to the predetermined speed.